[Issue 22522] New: [dip1000] Creating interior pointers allowed in @safe

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22522

          Issue ID: 22522
           Summary: [dip1000] Creating interior pointers allowed in @safe
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: dkorpel at live.nl

The garbage collection specification (https://dlang.org/spec/garbage.html)
mentions it's undefined behavior to have interior pointers in a struct:

> Do not have pointers in a struct instance that point back to the same instance. 
> The trouble with this is if the instance gets moved in memory, the pointer will 
> point back to where it came from, with likely disastrous results. 

Undefined behavior is not allowed in `@safe` code, but creating an interior
pointer is, which can break dip1000:

```
// compile with -preview=dip1000
@safe:
struct S {
    int storage;
    int* ptr;

    this(int dummy) {
        ptr = &storage;
    }

    int* get() return scope {
        return ptr;
    }
}

int* escape() {
    S s = S(0);
    return s.get; // escapes a pointer to stack variable `s`
}
```

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